1. Field of the Invention
The present invention relates to a thermal fixing device available in an image recording apparatus such as a copying machine, and more particularly to a thermal fixing device of the type wherein that a visible image formed on a recording medium (developer medium) by the chemical reaction is rendered stable by heating.
2. Description of the Related Art
In one conventional image recording apparatus, a recording medium is of a transfer type which uses a photosensitive microcapsule sheet and a developer sheet. More specifically, when using the transfer type recording medium, the developer material is coated on a separate substrate to provide a separate developer sheet or copy sheet. The photosensitive microcapsule sheet is provided with microcapsules which encapsulate therein a chromogenic material or dye precursor having first and second phases dependent on light exposure. The developer sheet is provided with a developer material which provides an output image upon reaction with the chromogenic material. Such an image recording medium is disclosed in, for example, U.S. Pat. No. 4,399,209.
As is well known, image recording apparatuses which use the transfer-type of recording media irradiate an original with light and a light reflected from the original or transmitted therethrough passes through an optical system such as a reflection mirror, a filter and lenses, and is focused onto the photosensitive microcapsule sheet. As a result, a latent image is formed on the photosensitive microcapsule sheet upon photocuring of the light-exposed microcapsules. Then, the light-exposed photosensitive microcapsule sheet is superposed with a developer sheet, and these sheets are pressed together at a pressure developing unit, so that non-photocured microcapsules are ruptured to permit the chromogenic material to react with the developer material, to thereby provide an output image on the developer sheet. After the pressure developing operation, the developer sheet is delivered to a thermal fixing device where the output image is thermally fixed and a glossy output image is obtained.
This type of thermal fixing device is constructed such that a thermal fixing belt is mounted tautly between a heat roller and a tensionally-acting roller, and a presser-roller is pressed against the heat roller with the thermal fixing belt interposed therebetween, followed by the movement of a recording medium according to the rotation of the thermal fixing belt, thereby causing an image formed on the recording medium to be heated and hence fixed thereon (see Japanese Patent Laid-Open No. 2-64640 and U.S. Ser. No. 07/398,979, filed on Aug. 28, 1989).
In this conventional thermal fixing device, however, the thermal fixing belt is always nipped between the heat roller and the presser-roller and is in a state in which it is locally pressed and deformed. Thus, when the deformed state of the thermal fixing belt is maintained at the same position over a long period of time, the deformed portion thereof is no longer returned to its original state. The conventional thermal fixing device is therefore accompanied by the problems that when a tension force is applied to the thermal fixing belt upon start-up of its rotation, for example, the thermal fixing belt runs idly to thereby cause a failure in feeding a recording medium or the thermal fixing belt tends to be cut due to the stress concentration on the deformed portion.